This is a D20 Modern version of the original Sidewinder rules, and from all accounts it provides an excellent Old West setting for adventures of all kinds.

Publisher's blurb: "Capture the cinematic action and adventure of your favorite western movies, TV shows and novels using this complete set of rules. Create courageous gunslingers, low-down rustlers or high-stakes gamblers and test your mettle out on the frontier."

More when I get hold of a copy!

Click HERE to buy this from RPG Now.
Or click HERE if you prefer a 'print on demand' copy from RPG Mall.

Book Details:
Authors: Eric Burns, Tom Gregory, Brad Hindman, Geoff Spakes and Chris Warner.
Publishers' Reference: DHR1000
ISBN: Unknown.
PDF or hardcopy, 294 pages
Date: February 2004